Various "dense wavelength-division multiplexing" (DWDM) solutions presently studied for the relief of capacity limitations in optical fiber communications systems include network elements with optical amplifiers for multiple-wavelength optical signals. Access to the DWDM system is provided at points where a single constituent wavelength is added to, or dropped from the DWDM path. A major obstacle in the way of implementing cost-effective add/drop access to individual wavelengths of the DWDM signal path has been the high cost of the optical amplifiers required on each single-wavelength input. In addition, the physical space and electrical power required by the optical amplifiers made it impossible to provide the number of single-wavelength inputs on one circuit pack that would be necessary to offer access to any one of N wavelength positions in a multi-wavelength input amplifier pack, where N&gt;1.
In Multi-wavelength Optical NETworks (MONET), per-wavelength constituent signals are cross-connected or locally accessed (add/drop). In the case of the composite multi-wavelength signals, high power optical amplifiers are typically used at each input and output of the system to boost the signal powers to the required value. In the case of the multiple single-wavelength access, multiple amplifiers would be required to individually boost each single wavelength signal level. This renders the system too costly.
Additionally, all optical communications systems including amplifiers or network elements, require that all signals entering and leaving the system be monitored for integrity. This helps in fault detection and localization of a detected fault. There are various techniques for performance monitoring (see for example, U.S. Pat. No. 5,317,439 issued May 31, 1994), but all require monitoring the integrity of the incoming and outgoing signals.